1. Field of the Invention
The present invention relates to a new variable focal length lens system and imaging apparatus. More specifically, the present invention relates to a variable focal length lens system whose zoom ratio exceeds 10× and that is used in video cameras, digital still cameras and the like, as well as to an imaging apparatus equipped with the above-mentioned variable focal length lens system.
2. Description of Related Art
There is known a method of recording a subject image, which is formed by an imaging device, as a recording means in a camera, using such photoelectric transducers as CCDs (Charge Coupled Devices) or CMOSs (Complementary Metal-Oxide Semiconductors) and formed on the plane of the imaging device, by converting the light intensity of the subject image into an electric output by way of each photoelectric transducer.
Along with the advancement in microfabrication technology in recent years, central processing units have become faster and storage media have witnessed higher integration. As a result it has become possible to process large volumes of image data, which could not be handled before, at high speeds. In addition, imaging devices have become more highly integrated and much smaller, and higher levels of integration have made recording at higher spatial frequencies possible, while their smaller sizes have brought about reductions in the overall size of cameras.
However, the above-mentioned integration and miniaturization result in a smaller light reception area for each photoelectric transducer, thereby causing a drop in the electric output which is accompanied by an increase in noise. In order to counter this issue, such attempts as increasing the light intensity that reaches the imaging device by magnifying the focal ratio of an optical system or placing small lens elements (referred to as micro lens arrays) right in front each photoelectric transducer have been made. While the above-mentioned micro lens arrays do guide beams of light that fall between adjacent photoelectric transducers onto the photoelectric transducers, they limit the positioning of the exit pupil in the lens system. In other words, as the exit pupil in the lens system approaches the imaging device, that is, as the angle between the chief ray reaching the imaging device and the optical axis become greater, off-axis light beams directed towards the periphery of the screen form greater angles with respect to the optical axis, and as a result, they do not reach the photoelectric transducers, and cause insufficient light intensity.
In recent years, as digital cameras have become more popular, users' needs have also become more diverse.
Zoom lenses that have a zoom ratio exceeding 10× are capable of capturing a subject in larger sizes. In particular, since cameras with integrated lenses cannot have their lenses changed, users of such cameras desire greater zoom ratios even if it means they have to settle for larger cameras.
As zoom lenses with zoom ratios exceeding 10×, those, for example, disclosed in Japanese Patent Application Publication Nos. 2005-215385 (Patent Document 1), 2003-295059 (Patent Document 2), 2005-128186 (Patent Document 3) and the like are known.
The zoom lenses disclosed in Patent Document 1 and Patent Document 2 include four lens groups, which are, in order and from the side of the object, a first lens group having a positive refractive power, a second lens group having a negative refractive power, a third lens group having a positive refractive power, and a fourth lens group having a positive refractive power. The four lens groups are so configured that when the lens position state changes from a wide-angle end state to a telephoto end state, the first lens group moves towards the object, the second lens group moves towards the image, the third lens group moves towards the image after first moving towards the object, and the fourth lens groups moves towards the image after first moving towards the object. In addition, the zoom lens disclosed in Patent Document 1 is so configured that an aperture stop placed between the second lens group and the third lens group moves independently of the other lens groups.
The zoom lens disclosed in Patent Document 3 includes four lens groups, a first lens group having a positive refractive power, a second lens group having a negative refractive power, a third lens group having a positive refractive power, and a fourth lens group having a positive refractive power. These lens groups are so configured that when the lens position state changes from the wide-angle end state to the telephoto end state, the first lens group and the third lens group are fixed in the direction of the optical axis, the second lens group moves towards the image, and the fourth lens group moves so as to compensate for changes in the position of the plane of the image that accompany the movement of the second lens group.